Bead thermistor



Feb. 22, 1949. F. H. DRAKE ETAL BEAD THERMISTOR 2 Sheets-Sheet 1 FiledOct. 10, 1946 Feb- 22, l'949- F. H. BRAKE E-rAL BEAD THERIISTCR 2shuts-Sheet 2 Filed Oct. 10, 1946 Patented Feb. 1949 BEAD THERMISTORFrederick H. Drake, Boonton, and Paul B. King,

Jr., Mountain Lakes, N. J.

, signora to Aircraft Radio Corporation, Boonton, N. J., a corporationof New Jersey Application October 10, 1948, Serial No. 702,378 14Claims. (Cl. 201-63) 1 This invention relates to bead thermistors, andmore particularly mounted thermlstors and to methods of assembling thesame.

The name thermistor is a contraction oi thermally sensitive resistor andidentities a circuit element in which the electrical resistance varieswidely with temperature. Thermistors are manufactured as disks, rods orbeads, and the present invention relates to bead thermistors such asemployed in sensitive current and power measuring circuits. In thisapplication, the unknown power to be measured is dissipated in thethermistor with consequent rise in temperature, and the thermistorresistance which is dependent upon its temperaturethus allords a measureof the power dissipated.

Objects ot the invention are to provide mounted thermistor assemblies ofrelatively small size, and to methods of assembling the same to obtainsubstantial uniformity in operating characteristics. An object is toprovide mounted thermistors which are characterized by a simpleleak-prooi' construction and a quarter-wavelength isolation of thethermistor. A further object is to provide a thermistor mounting whichincludes an adlustable element for controlling the'operatingcharacteristics of the thermistor.

These and other objects and advantages of the invention will be apparentfrom the following speciiication when taken with the accompanyingdrawing in which:

Figs. 1 and 2 are perspective views of la ther- 4 mistor embodying theinvention, the views being from opposite ends of the housing of thethermistor;

Fig. 3 is a longitudinal section on a larger scale and as seen on theplane of line 3-3 oi' Fig. 4;

Fig. 4 is a side elevation, with parts in section as seen upon the planeof line 4-4 of Fig. 3:

Fig. 5 is an exploded perspective view oi.' the assembly which providesan insulated support for one of the wires upon which the thermistor beadis mounted;

Fig. 6 is an elevation of the assembly;

Fig. 7 is an elevation, with parts in section, of a tool or jig on whichthe elements of a thermistor mount are supported in position to receivea thermistor element and Figs. 8 and 9 are fragmentary elevations on alarge scale illustrating the thermistor element and its supporting wiresat diilerent stages of the assembly operation.

In the drawings. the reference numeral l identiiles a short section ofwave guide tubing which is closed at one end by a wall 2 and which isProvided at its other end with any desired form of coupling l forsecuring the thermistor mounting to a wave guide system. Opposite wallsof the tubing I have alined apertures over which a cap 4 and a bushingl, respectively, are rigidly secured by soldering or brazing. The cap 4and bushing i have tubular extensions which tit snugly within theapertures ci the wave guide tube l, and these extensions are accuratelysized to locate the inner ends o! the cap 4 and the bushing I in theplanes of the opposed inner wall surfaces ot the wave guide tube I. Thecap 4 is counterbored from its inner end to provide a cylindrical recess4' having a length, measured from the inner surface of the wave guidetube l. oi M4, i. e., onefourth oi the wavelength of the microwavegenerator or transmission system with which the thermistor is to beused. Similarly, the inner end of the bushing 5 has a cylindrical borei' which opens into the wave guide tube l, the eil'ective length of thebore as measured from the inner surface of the wave guide being M4.These desired dimensions for the pockets 4', l are readily attainedduring manufacture ot the cap 4 and bushing 5 when, as described above,the cap 4 and bushing 5 are formed with tubular extensions which passthrough and terminate flush with the inner surfaces of the walls of thewave guide tube l.

The wires l, 'l which carry a thermistor T extend axially through thepockets 4', 5' respectively, and are supported by the cap 4 and bushingl. The method of mounting the thermistor on wires i, 'l to insure theaccurate location of the thermistor bead at the center of the wave guidetube l will be described later.

The wire 5 is soldered to the cap 4 and the other wire 'l extendsthrough the bore ci' a. metal sleeve 8 which is axially arranged withrespect to the outer counterbored section of the bushing 5. The innerend of the sleeve 8 terminates in a radial ilange or head 9 whichextends across the outer end of the pocket or bore 5' of the bushing 5,and the head 9 is insulated from the bushing 5 by a mica washer I0 of apreselected thickness which locates the inner face of the sleeve head 9at the desired )./4 distance from the wave guide passage. A powderediron choke ll surrounds the sleeve 8 and is seated against the outerface of the head 9, the choke being of somewhat less diameter than thecounterbore of the bushing 5. Mica washers l2 are iitted over the outerend of the sleeve 8, the washers being oi approximately the diameter ofthe outer end ot the bushing 5 but having notches at their edges toclear the end projections I3 of the bushing. A metal washer I4 isarranged outside the mica washers I2 to prevent damage to the same whenthe choke II and washers I2 are rigidly assembled on the sleeve 8 byriveting or spinning a sleeve flange I5 over the washer I4. The lengthof the bushing extensions I3 is such that the washers I2 do not seatagainst the outer end of the bushing when the cap I6 is turned down uponthe bushing to clamp the sleeve head 9 against the mica washer I0. Thenotches in the washers I2 iit about the extensions I3 to preventrotation of the sleeve and core assembly within the bushing.

A compensating disk thermistor T' is mounted in a socket II which issoldered or brazed to the tube I, and isv resiliently supported betweenspring contact fingers I8 which are riveted or welded to the inner endwall of the socket and similar spring contact fingers I9 secured to amica washer or cover plate 20 by spinning over the inner end of aterminal 2| which projects through and has a Ilange seated against theouter face of the mica cover plate 20. The electrical connection of thethermistor disk T between the tube I and the terminal 2| is therebyeffected automatically when a cover 22 is turned down upon the threadedsocket I1. The terminal 2| is to be connected into the power measuringnetwork in the usual manner.

The axis of the thermistor supporting wires 6, 1 is spaced from the endwall 2 of the Wave guide tube by somewhat less than one-half thewavelength of the microwave system with which the thermistor assembly isto be used. Compensation for manufacturing errors is obtained bythreading a screw 23 into the wave guide between the end wall 2 and theaxis of the thermistor supporting wires. The projection of thecompensating screw 23 into the wave guide has the same electrical eiTectas a displacement of the end wall 2 away from the thermistor T, and thedesired electrical characteristics of the thermistor may therefore beobtained by turning the screw 23 into or out of the wave guide passage.The screw 23 is fixed in any desired adjusted position by staking thecap 4, as indicated by the recess 24, to interlock with the screw 23 toprevent further adjustment thereof. The outer end of the screw 23, i.e., the portion shown inv dotted lines, is then cut olf.

The described apparatus is preferably manufacturedA or assembled in thefollowing manner to insure the location of the thermistor T at thegeometrical center of a transverse plane through the wave guide tube I.The pocket 5 of the bushing 5 is of a predetermined axial length, andthe position of the inner end of the supporting wire 'I within the tubeI is determined by the projection of the wire 'I beyond the head 9 ofthe sleeve 8 and the thickness of the washer I0. The wire I is insertedin the sleeve 8 and the sleeve head 9 is staked, as indicated by therecesses 25, and soldered to the sleeve. This operation may be eiectedin av jig or tool, not shown, which locates the inner end, i. e. theleft end as shown in Fig. 7, of the wire I with respect to the face ofthe head 9. Alternatively, the parts may be assembled and interlockedwith the inner end of the wire 'I projecting an arbitrary excessdistance beyond the head 9. The excess length is then ground oi' orfiled off to the exact predetermined length. The choke I I, insulatingwasher I2 and metal washer I4 are then assembled on the sleeve 8 andsecured thereto by riveting, spiniirm clamping engagement with thewasher I4.

The washer I0 is placed within the bushing 5, and the sleeve 8 assemblyis slipped into the bushing and the cap I6 is turned down loosely on thebushing to hold the parts in assembled relation.

The casing or wave guide tube is then clamped in a tool or jig which isshown schematically in Fig. 7 as a base 28 with end plates 21, 28, and abridge 29 slidable on the base 26. The end plates 21, 28 have alined pinclamps 30, 3|, respectively, in which the outer end of the sleeve 8 andthe thermistor supporting wire 6 may be clamped. The casing is mountedon the tool or jig by inserting the barrel of the bushing 5in theopening of the bridge 29, and is clamped to the bridge by appropriatemeans which is shown schematically inthe drawings as a screw 32. Thebridge 29 is then moved to the .right to project the outer end of thesleeve 8 into the pin clamp 30, and the clamp is adjusted to grip thesleeve 8. The

cap I 6 is then backed off and the bridge 29 is moved to the left toclear the inner end of the wire, and the wire 6 is passed through thepin clutch 3| and the cap 4 of the thermistor case. end of the wire 6 isaccurately adjusted to a predetermined distance from the inner end ofthe wire 'I by a thickness gage, not shown. The pin clutch 3| is clampedupon the Wire 6 to maintain the desired spacing, and the leads of thethermistor T are then spot welded to the adjacent ends of the wires 6,I, see Fig. 8. An appreciable amount of slack is left in the leads ofthe thermistor 'I' to permit the adjustment of the thermistor bead intoa central position at the axis of the supporting wires 6, I, see Fig. 9.The free ends of the thermistor leads are then cut oif or broken off,and the bridge 29 is moved to the right to seat the sleeve head 9against the insulating washer IIJ. The cap I6 is turned down firmly uponthe bushing 5 to seal the outer end of the bushing by clampingengagement with the insulating washers I2. Pin clamps 30, 3i arereleased, and the clamp screw 32 is backed oif to permit removal of thethermistor casing from the assembly tool. The excess length of the Wire6 is cut olf, and the outer end of the wire 6 is soldered to the cap 4of the thermistor casing.

The bead of the thermistor T is accurately located at the axis of thewave guide tube by this process of assembly since the inner ends of thewires 6, 'I are symmetrically positioned with respect to the tube axis,and the thermistor bead is located centrally of the short gap betweenthe wires 6 and I. The mounted thermistor is then connected into aCalibrating system, the screw 23 is adjusted to obtain a desiredelectrical elect, and the excess length of the screw 23 is out off.

It is to be noted that the thermistor assembly is of relatively smallsize as the effective isolation of the direct current measuring circuitfrom the wave guide system is obtained by the pockets 4', 5' which eachhave an axial length of one-fourth of the wavelength of the microwavesystem.

It is to be understood that the invention is not limited to theparticular embodiment herein shown and described as Various changeswhich may occur to those familiar with the design and construction ofthermistors fall within the spirit and scope of the invention as setforth in the following claims.

We claim:

1. A mounted thermistor comprising a wave means at the other end, saidtube having alined The inner l openings in opposite walls thereof.`cover members gap between the adjacent inner ends oi the wires,

means sealing the outer ends or each wire in its supporting covermember, said sealing means insulating one wire from its cover member.

2. A mounted thermistor as recited in claim 1.'

wherein the axis oi the thermistor supporting wires is spaced from theclosed end of said wave guide tube by somewhat less than one-halfwavelength. in combination with means mounted on said wave guide tubeadJacent the closed end thereof and adjustable to project into the tubeto determine an electrical characteristic of the mounted thermistor.

3. A mounted thermistor as recited in claim 1, wherein the axis of thethermistor supporting wires is spaced from the closed end of said waveguide tube by somewhat less than one-half wavelength, in combinationwith means mounted on said wave guide tube adjacent the closed endthereof and adjustable to project into the tube to determine anelectrical characteristic of the mounted thermistor, and means lockingsaid last mentioned means in a desired position of adjustment.

4. A mounted thermistor as recited in claim l,

i in combination with a socket secured to said wave guide tube toreceive a compensating disk thermistor, a cover of insulating materialand means securing the same over said socket, a terminal mounted on saidsocket cover, and resilient contact means for engagement rwith theopposite faces of a disk thermistor to connect the same between thesocket and said terminal when the cover is secured over said socket.

5. A mounted thermistor as recited in claim 1. in combination with asocket secured to said wave guide tube, resilient contact means at thebase of said socket, a compensating disk thermistor in said socket andseated on said resilient contact means, a terminal having resilientIcontact means at its inner end to seat upon the outer face of the diskthermistor, and means securing said terminal to and insulating the samefrom said socket.

6. A mounted thermistor as recited in claim 1, wherein the cover membersproject into said openings of the wave guide tube and terminate in therespective planes of the inner surfaces of the opposite walls thereof.

7. A mounted thermistor comprising a wave guide tube having openings inits opposite walls aiined transversely of the axis of the tube, a capcovering one opening and recessed from its inner face to provide aquarter wavelength pocket, a bushing secured over t-he other opening,said bushing having an inner bore with an axial length of aquarter-wavelength and lbeing counterbored `from its outer end, aninsulating washer at the inner end of the counterbore, a hollow sleevehaving a head seated against said washer, a magnetic material choke onsaid sleeve, means securing said sleeve and choke within and insulatingthe same from said bushing, a thermistor positioned substantiallyaxially of said tube. and supporting wires for said thermistor extendingthrough and secured to said cap and said sleeve respectively, the wirewhich extends through the cap being electrically connected thereto. 8. Amounted thermistor as recited in claim 7, wherein said cap has an endportion extending into its associated wave guide tube opening andterminating ilush with the inner surface oi the tube wall, whereby thequarter-wavelength pocket may be accurately formed in the cap prior toits assembly on the wave guide tube.

9. A mounted thermistor as claimed in claim 7,

-wherein said bushing has an end portion extending into its associatedwave guide tube opening and terminating ilush with the inner surface oithe tube wall, whereby the quarter-wavelength bore in the bushing may beaccurately formed prior to the assembly oi the bushing on the wave cuidetube.

10. The process of mounting a thermistor in a wave luide tube havingopenings in opposite walls over which a cap and a bushing respectivelyare secured, said process comprising securing a thermistor supportingwire to means for mounting the same within the bushing and with theinner end oi the wire projecting a predetermined distance beyond saidmounting means to extend approximately to but Short of the axis of thewave guide tube, inserting said wire and its mounting means in saidbushing, supporting said wave guide tube and said wire mounting meansfor relative movement axially of the supporting wire, displacing saidtube and supporting means to position the inner end of the supportingWire outside of said bushing, inserting a second supporting wire throughsaid cap and positioning its inner end in predetermined spaced relationto the ilrst supporting wire, welding the leads of a thermistor to theadjacent ends of the supporting wires, displacing said tube andsupporting means to position the supporting means Within the f` bushing,and removing the excess length of the second supporting wire whichprojects beyond said cap.

11. The process as recited in claim 10, in combination with the stepofsoldering the second supporting wire to said cap.

12. The process ci' mounting a bead thermistor in a Wave guide tubehaving a cap and bushing secured over openings in the opposite walls ofa wave guide tube, the cap having an axial bore alined with the bore ofthe bushing; said process comprising securing a thermistor supportingwire in the bore of a headed hollow sleeve adapted to be mounted in thebushing, accurately determining the length of the inner end of thesupporting wire projecting beyond said sleeve, positioning the waveguide tube and the sleeve with the bore of the cap axially alined withthe supporting wire and with the inner end of the supporting wireoutside of said bushing, inserting a second supporting wire through thebore of the cap and securing the same with the inner end thereof inpredetermined spaced relation to the aiined inner end of the ilrstsupporting wire, welding the leads of a thermistor bead to the adjacentends of the supporting wires, effecting relative displacement of thewave guide tube and the axially alined supporting wires to position thesleeve within the bushing, and securing the sleeve Within the bushing.

13. The process as recited in claim 12, wherein the supporting wires aresecured in iixed positions, and the wave guide tube isl displaced withrespect thereto to position the sleeve within the bushing. V-

7 l 14. The process s recited in claim 12, wherein EWR/ENOES Cl'lm slackis left ln the vleads of the thermistor in the welding stephinombination with the step of mhf fligwlg :gemmes are of record in theadjusting the thermistor bead into substantially p en central posmon'mthe gapxbetwee'n she @uned 5 y, :Umm STATES mms inner ends of thesupporting wlrs. Number Name Date FR En 2,402,663 0111 June 25, 1943ERICE H' BRAKE' $13,021 Wolfson et al. Dec. 24, 1946 PAUL B. KING, JR.

